Electrophotography is a useful process for printing images on a receiver (or “imaging substrate”), such as a piece or sheet of paper or another planar medium, glass, fabric, metal, or other objects as will be described below. In this process, an electrostatic latent image is formed on a photoreceptor by uniformly charging the photoreceptor and then discharging selected areas of the uniform charge to yield an electrostatic charge pattern corresponding to the desired image (a “latent image”).
After the latent image is formed, charged toner particles are brought into the vicinity of the photoreceptor and are attracted to the latent image to develop the latent image into a visible image. Note that the visible image may not be visible to the naked eye depending on the composition of the toner particles (e.g. clear toner).
After the latent image is developed into a visible image on the photoreceptor, a suitable receiver is brought into juxtaposition with the visible image. A suitable electric field is applied to transfer the toner particles of the visible image to the receiver to form the desired print image on the receiver. The imaging process is typically repeated many times with reusable photoreceptors.
The receiver is then removed from its operative association with the photoreceptor and subjected to heat or pressure to permanently fix (“fuse”) the print image to the receiver. Plural print images, e.g. of separations of different colors, are overlaid on one receiver before fusing to form a multi-color print image on the receiver.
Electrophotographic (EP) printers typically transport the receiver past the photoreceptor to form the print image. The direction of travel of the receiver is referred to as the slow-scan, process, or in-track direction. This is typically the vertical (Y) direction of a portrait-oriented receiver. The direction perpendicular to the slow-scan direction is referred to as the fast-scan, cross-process, or cross-track direction, and is typically the horizontal (X) direction of a portrait-oriented receiver. “Scan” does not imply that any components are moving or scanning across the receiver; the terminology is conventional in the art.
Toner is transferred between members in the printer using electrostatic forces. Variations in the electrical properties of transferring members will result in variations in transfer efficiency. These variations can cause an incorrect amount of toner to be transferred, producing nonuniformities and reducing image quality. Moreover, variations over time can gradually degrade overall transfer performance, resulting in prints that do not consistently produce the expected density.
Various schemes have been proposed to deal with these problems. For example, U.S. Pat. No. 7,742,729 to Sawai describes selecting a transfer member which changes resistance sufficiently slowly to remain within an acceptable range over the printing of 200,000-300,000 copies. Sawai also describes testing a transfer member by cycling voltage across an intermediate transfer medium. However, this testing requires mechanical contact with the intermediate transfer medium, which can lead to increased contamination on the surface of the transfer member or other members if this test is performed in the printer. In related schemes, resistance changes of transferring members have been measured by measurement rollers brought into contact with those members. However, these schemes can also lead to increased wear and contamination on the surface of the members. Another scheme, U.S. Pat. No. 5,953,556 to Yamanaka, describes measuring transfer current through the transfer member and transfer voltage to determine resistance. However, the results of this method are affected by the toner pattern being printed. As printers move towards higher printing speeds and smaller lead edge margins, including full-page bleed printing, there is insufficient time available to measure resistance in a non-print region. The resistance measurement will also depend upon the properties of the photoreceptor, which may change with time. For example, the thickness of a photoreceptor typically decreases with age due to abrasion of a blade cleaner.
There is a continuing need, therefore, for a way of measuring the electrical properties of members that transfer toner.